One of the most basic requirements for any cellular system is the possibility for a node to initially request a connection setup, commonly referred to as random access. In LTE, the random access procedure comes in two forms, allowing access to be either contention-based or contention-free. The contention-based procedure as initiated by a node consists of four-steps, 1) Preamble transmission, 2) Random access response, 3) Layer 2/Layer 3 message transmission; 4) Contention resolution message.
In LTE random access preambles in step 1) are generated from cyclic shifts of one or several root Zadoff-Chu (ZC) sequence. Basically there are a number of available random access preambles for the node to select to do the random access in one cell. E.g. in an LTE system, there are 64 random access preambles that can be used for random access in each cell. Each time when the node is about to do the random access, one random access preamble out of the 64 random access preambles is selected. A collision will occur if several nodes are selecting the same random access preamble which could result in random access failure for some or all nodes. It is therefore important that the multiple nodes are configured to choose the same random access preamble with a low probability. The larger the number of different random access preambles that are available, the smaller the probability of random access failure due to collision.
In LTE, once a random access preamble is detected, the access node would send a Random Access Response (RAR) on the Physical Downlink Shared Channel, PDSCH, in step 2), and address the node with a Random Access Radio Network Temporary Identifier (RA-LNTI), which conveys the identity of the detected random access preamble, a timing alignment instruction to synchronize subsequent uplink transmission from the node, an initial uplink resource grant for transmission of the Step 3 message, and an assignment of a temporary Cell Radio Network Temporary Identifier (C-RNTI).
In Step 3, the node would convey the random access procedure message, such as an RRC connection request, tracking area update, or scheduling request. Finally, in the following Step 4, the contention resolution message would be sent by access node.
To work against phase noise and frequency error for higher carrier frequencies and reduce the hardware complexity for multiple antennas, a new random access preamble is proposed. The proposed random access preamble is constructed by repeating a short sequence multiple times and a corresponding random access preamble detector reusing the FFTs for other uplink channels and signals. In this way, the amount of special random-access related processing and hardware support is significantly reduced for multi-antenna systems, and the detector is also robust against inter-carrier interference from other uplink channels and signals. Furthermore, the proposed random access preamble detector scheme can be used in scenarios with a high amount of phase noise and frequency errors.
From a frequency point of view, the random access preamble would occupy a number of RB (Resource Blocks) or equivalently a number of sub-carriers. Here the length of the frequency allocation should be a prime number in order to use Zadoff-Chu sequences which are known to have good auto- and cross-correlation properties. This prime number must be less than, or equal to, the number of allocated sub-carriers.
For example, a random access allocation similar to the one used in LTE might be used where the preamble occupies six RBs, where each RB contains twelve sub-carriers. The sub-carriers index for the six RBs could be m, m+1, . . . , m+71, where m is the starting position or starting sub-carrier index for the six RBs.
In general, the number of available random access preambles can be increased by increasing the frequency allocation a random access channel on the basis that a larger frequency allocation allows for longer root sequences to be used.
However, the number of available random access preambles only increases linearly with the number of allocated sub-carriers.